Organ procurement currently poses one of the major problems in solid organ transplantation, since the number of patients requiring transplants far exceeds the number of organs available. One means of eliminating the shortage of donor organs for allotransplantation is to develop the technologies required to transplant non-human organs into humans, i.e., xenotransplantation. The development of clinical xenotransplantation will also allow for the transplantation of non-human cells and tissues.
A potential problem lies in the fact that human and animal organs may be of very different size, depending on the species serving as donor, and on the possibility of infection due to microorganisms present in the donor tissues and having an ability to infect humans. Consequently, one strain of the domesticated pig, denoted miniature swine (Sus scrofa), appears suitable for such transplants because of its similar size to humans (see below). Furthermore, any use of pigs as organ donors in xenotransplantation would obviate problems associated with the consideration of non-human primates as donors. Xenografts from non-human primates, for example, present considerable risk of transmission of pathogens and the consequent development of emerging infections. In addition, several pathogens that cause disease are known to infect both humans and non-human primates, for example, in the transmission of HIV from the chimpanzee to humans. Furthermore, chimpanzees and orangutans, the closest non-human primates phylogenetically, are endangered species and far too rare to be considered for organ transplantation purposes. Baboons are too small to be an appropriate donor for most organ transplants. Even the largest baboons weigh less than 40 kg. In addition, the gestation times and productivity of primates would not allow a commercially significant generation of source animals.
The physiology of many organ systems of pigs has been shown to be highly similar to the human counterparts (Sachs, D. H. (1994) Veterinary Immunology & Immunopathology 43:185-191). Thus, the miniature swine offers numerous advantages as potential xenograft donors. They achieve adult weights of approximately 100-150 kg, a size that is more compatible to human weights than that of the domestic pig, which reaches weights of over 500 kg. Through a selective breeding program over the past 20 years, partially inbred, miniature swine have been produced (Sachs et al. (1976) Transplantation 22: 559-567; Sachs, D. H. (1992) In Swine as models in biomedical research, eds M. Swindle, D. Moody, and L. Phillips, pp. 3-15. Ames Iowa State Univ. Press; Sachs, (1994) Veterinary Immunology & Immunopathology 43: 185-191). This breeding program has resulted in herds of animals that are genetically well characterized and inbred at the major histocompatibility complex (MHC). These animals have been used in large animal model studies for many years and have, like their domestic counterparts, very favorable breeding characteristics for being used as donors of organs in xenotransplantation.
A central concern regarding xenotransplantation is the risk of xenosis, infection by organisms transferred with the xenograft into both the transplant recipient and the general population. In particular, “emerging infections” caused by new and previously unknown infectious agents with altered pathogenicity, have to be considered as a potential risk associated with xenotransplantation. The risk of viral infection is increased in transplantation by the presence of factors commonly associated with viral activation, e.g., immune suppression, graft-versus-host disease, graft rejection, viral co-infection, and cytotoxic therapies.
Herpesviruses are the causative agents of many diseases that share a commonality of latency and recurrent infections. Herpesviruses may persist for years in a dormant state and become reactivated after later provocation. While the herpesviruses are widely separated in terms of genomic sequence and proteins, many are similar in terms of virion structure and genome organization. Herpesvirus represents a DNA virus family containing a central icosahedral core of double-stranded DNA. There is a lipoprotein envelope that is trilaminar and 100-200 nm in diameter and a nucleus that is 30-43 nm in diameter. The genome size is large, up to 235 kbp DNA. Based upon the structural and morphological features, the herpesvirus family is divided into three main families: alpha, beta, and gamma. Examples of alpha herpesviruses are herpes simplex and varicella zoster, examples of beta herpesviruses are cytomegalovirus and human herpesvirus 6 while examples of gamma-herpesviruses are Epstein Barr virus and human herpesvirus 8.
Prior to this invention, members of three porcine herpesvirus families had been identified, namely of the alpha, beta, and gamma-herpesvirus families. Suid herpesvirus 1 (SHV1), which causes pseudorabies (PRV) in pigs, is an alpha-herpesvirus and results in neonatal death of piglets, and can be eradicated by vaccination. The glycoprotein II gene of SHV1 is reportedly closely related to the gpB gene of other herpesviruses (Robbins et al. (1987) J. Virology. 61:2691-2701). Suid herpesvirus 2 (SHV2), also known as pig cytomegalovirus (pCMV), is found in the respiratory tract of pigs and causes atopic rhinitis, abortion, or neonatal piglet losses. Only the DNA polymerase gene of SHV2 has been reported (Genbank Accession Number AJ222640). Detection of two novel porcine herpesviruses with high similarity to other gamma-herpesviruses were recently reported (Ehlers et al. (1999) J. General Virology, 80:971-978), wherein the sequence of the DNA polymerase gene was reported (Genbank Accession Numbers AF118399 and AF118401).
Subsequent examination, as disclosed herein, of pigs for the presence of a gamma-herpesvirus by PCR methods designed to amplify the DNA regions encoding all or part of the glycoprotein B (gpB) envelope molecule has resulted in the detection of sequence similarity to other known gamma-herpesviruses.